The following patents and/or commonly assigned patent applications are hereby incorporated herein by reference:
U.S. Pat. Nos. 5,061,049 5,583,688
Filing Date Sep. 13, 1990 Dec. 21, 1993
Issue Date Oct. 29, 1991 Dec. 10, 1996
Title Spatial Light Modulator and Method Multi-Level Digital Micromirror Device
This invention relates to the field of micromirror display systems, more particularly to micromirror architectures designed to minimize the size of image projection systems.
Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits.
Digital micromirror devices (DMDs), sometimes referred to as deformable micromirror devices, are a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and DMDs have found commercial success, other types have not yet been commercially viable.
Digital micromirror devices are primarily used in optical display systems. In display systems, the DMD is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, DMDs typically operate in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schlieren optics illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface: concentrating the deformation on a relatively small portion of the DMD surface. The rigid mirror remains flat while the hinges deform, minimizing the amount of light scattered by the device and improving the contrast ratio of the device.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by fabricating the mirror on a pedestal above the torsion beams. The elevated mirror covers the torsion beams, torsion beam supports, and a rigid yoke connecting the torsion beams and mirror support, further improving the contrast ratio of images produced by the device. The small size of the micromirror array has enabled a remarkable reduction in the size and weight of image projectors. Projectors are now sold that weigh less than five pounds. Consumers desire yet further reductions in projector size and weight. New optical designs are needed to enable further reductions in the size and weight of image projectors.
Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention which provides a method and system for orthogonal illumination of micromirror devices. One embodiment of the claimed invention provides a micromirror array designed for orthogonal illumination. The micromirror comprises: a substrate, addressing circuitry formed on the substrate, and an array of deflectable mirror elements formed over the substrate. The deflectable mirror elements rotate about an axis that is substantially parallel to at least one of a leading and trailing edge in response to the addressing circuitry. At least one of the leading and trailing edgesxe2x80x94preferably bothxe2x80x94are jagged. The jagged edge is typically formed by a series of saw tooth shapes. Three saw teeth provide a sufficiently jagged edge without complicating the fabrication of the micromirrors.
The jagged leading and trailing edges reduce the diffraction that would occur when using orthogonal illuminationxe2x80x94that is, when the illumination axis is perpendicular to the axis of rotation. Orthogonal illumination requires a much smaller prism than the 45xc2x0 illumination of the prior art. The smaller prism lowers the cost, size, and weight of the display system compared to the larger prisms of the prior art.